Ceramic articles have many uses including catalyst supports, dental porcelain, heat exchangers, turbine blades, substrates for integrated circuits, etc. One of the areas of increasing interest is the use of ceramic articles as electronic substrate materials. To be useful as an electronic substrate, a ceramic material should have a thermal expansion that matches that of silicon metal, low dielectric constant, low electronic conductivity, high thermal conductivity and low sintering temperature. Cordierite is one ceramic material that meets most of these criteria. However, the conventional manner of preparing cordierite involves sintering at temperatures above 1250.degree. C. and the resultant material does not have as high a thermal conductivity as desired. Therefore, there is a need for a process to prepare a high density cordierite at as low a temperature as possible and which has as high a thermal conductivity as possible.
The prior art reveals several ways to prepare cordierite other than the conventional method of sintering the oxides. For example, J. R. Moyer et al. in "Synthesis of Oxide Ceramic Powders by Aqueous Coprecipitation", Materials Research Society Symposium, Vol. 73, p. 117 (1986) discloses preparing cordierite by coprecipitating aluminum, magnesium and silicon compounds and then firing. However, sintering must be carried out at about 1420.degree. C. in order to obtain a cordierite article with a density of at least 97% of its theoretical density.
The prior art also discloses that zeolites can be used to form cordierite. Thus, D. W. Breck in ZEOLITE MOLECULAR SIEVES, John Wiley & Sons, New York (1974), pp. 493-496 states that Mg-X can be heated to form cordierite. The disclosed process involves heating the Mg-X zeolite at 1500.degree. C. to form a glass and then heating the glass above 1000.degree. C. to form cordierite. Thus, two steps are required to form cordierite.
Another reference which teaches the preparation of a cordierite based ceramic article is U.S. Pat. No. 4,814,303 to Chowdry et al. Chowdry discloses producing a monolithic anorthite, anorthite-cordierite or cordierite based ceramic article by heating the Ca, Ca/Mg and Mg forms of zeolites X, Y and A at a temperature of about 900.degree. C. to about 1350.degree. C. Chowdry also discloses that one should maximize the removal of sodium present in the zeolites since sodium ions are known to significantly increase the dielectric constant and dielectric loss.
Finally, European Patent Publication Number 298,701 describes the preparation of a ceramic article having an anorthite phase from a calcium zeolite. The process involves a calcination to form an amorphous product which can then be shaped into an article and sintered at temperatures of about 850.degree.-950.degree. C.
In contrast to this prior art, applicants have discovered that heating a magnesium exchanged phillipsite family zeolite at a temperature of about 1000.degree.-1050.degree. C. produces a ceramic article in which the only crystalline phase is cordierite and which has a density of at least 90% of the theoretical density of cordierite which is 2.51 g/cc. The phillipsite family of zeolite are those natural and synthetic zeolites which have similar framework structures and consequently similar X-ray diffraction patterns but which may have different overall crystal symmetry and chemical composition, e.g., SiO.sub.2 /Al.sub.2 O.sub.3 ratio, cation type and content, water content, etc. The phillipsite family of zeolites are phillipsite, harmotone, gismondine, zeolite B (also known as zeolite P), zeolite ZK-19 and zeolite W.
The advantage to using a Mg phillipsite family zeolite over the Mg-X, Y or A disclosed by Chowdry is that the sodium which is present in the phillipsite family zeolite can be removed much more easily than the sodium in zeolite X, Y or A. This is especially true of zeolites B and W. Therefore, there is an economic advantage to using a phillipsite family zeolite, and especially zeolites B and W over zeolite X, Y or A. Additionally when zeolite X, Y or A is heated at about 1000.degree. C. one obtains a mixture of phases which includes sapphrine, spinel, stuffed beta quartz and mullite, whereas when a phillipsite family zeolite is heated at 1000.degree. C. one only obtains a cordierite phase or a mixture of cordierite and minor amounts of stuffed beta quartz which is a metastable precursor of cordierite (also known as mu-cordierite.) It should be noted that when cordierite is used in this application what is meant is alpha cordierite which has the diffraction pattern set forth in JCPDS #9-472. Finally, the density of a ceramic article prepared by using a phillipsite family zeolite is much higher than that formed using zeolite X, Y and A. Accordingly, applicants have shown unexpected results by using a phillipsite family zeolite, and especially Mg-B zeolite, instead of Mg-X, Y or A.
Applicants have also discovered that cracks in the finished ceramic article can be minimized by first heating a phillipsite family zeolite at a temperature of about 600.degree.-800.degree. C. This heating collapses the zeolite structure, thereby minimizing crack formation and/or propagation. Applicant's process is different from that set forth in Eup. Publ. No. 298,701 in that first applicants are preparing a crack free cordierite ceramic article, whereas the '701 reference involves a process for preparing an anorthite ceramic. Accordingly, the calcination temperature (800.degree.-900.degree. C. disclosed in the '701 reference would initiate crystallization of cordierite, whereas the final sintering temperature disclosed (850.degree.-950.degree. C.) is not sufficiently high to form a high density, crack free cordierite ceramic article. Therefore, there is nothing in the '701 reference to suggest that the process disclosed therein could be used to prepare a crack free cordierite ceramic article.
Further, while Breck at pp. 493-94 of ZEOLITE MOLECULAR SIEVES, teaches a two-step process to make cordierite from Mg-X, the steps involve heating at 1500.degree. C. to form a glass, followed by heating at 1000.degree. C. to form cordierite; there is no mention in Breck that one should collapse the zeolite structure prior to preparing a shaped article in order to minimize crack formation. In fact, Breck does not address the preparation of ceramic articles at all but speaks only about converting Mg-X zeolite to cordierite. Further, Chowdry specifically states at column 3, lines 40-46 that the advantage to their invention is a single firing step. Clearly, applicants are the first to recognize that precollapsing the zeolite framework minimizes cracks in the final ceramic article.
Applicants have additionally discovered that the ceramic article obtained by the present invention has unique properties which are an average grain size of about 10 to about 500 nanometers and a Vickers hardness of about 5 to about 8 GPa. Grain sizes this small have never been observed for a high density cordierite material. This small grain size makes the cordierite ceramic article very ductile.